feat(videos): wire BackgroundMusic into all 3 promo compositions

Tracks from Mixkit (royalty-free, no attribution required):
- PromoTeaser: "Life's a Movie" by Arulo (50% vol)
- XLaunchVideo: "Techno Fest Vibes" by A.M. (40% vol)
- ProductDemo: "Rising Forest" by Diego Nava (30% vol)

apps/videos/src/compositions/product-demo/ProductDemo.tsx

@@ -1,6 +1,7 @@
 import type React from "react";
-import { AbsoluteFill, Series } from "remotion";
+import { AbsoluteFill, Series, staticFile } from "remotion";
 import { GrainOverlay } from "@/components/GrainOverlay";
+import { BackgroundMusic } from "@/lib/audio";
 import { AiToolsScene } from "./scenes/AiToolsScene";
 import { ApiDocsScene } from "./scenes/ApiDocsScene";
 import { BatchProcessingScene } from "./scenes/BatchProcessingScene";
@@ -41,6 +42,13 @@ export const ProductDemo: React.FC = () => {
       </Series>
 
       <GrainOverlay opacity={0.02} />
+      <BackgroundMusic
+        src={staticFile("audio/product-demo.mp3")}
+        volume={0.3}
+        fadeInFrames={30}
+        fadeOutFrames={60}
+        totalFrames={2250}
+      />
     </AbsoluteFill>
   );
 };

apps/videos/src/compositions/promo-teaser/PromoTeaser.tsx

@@ -1,7 +1,8 @@
 import type React from "react";
-import { AbsoluteFill, Series } from "remotion";
+import { AbsoluteFill, Series, staticFile } from "remotion";
 import { GradientBlob } from "@/components/GradientBlob";
 import { GrainOverlay } from "@/components/GrainOverlay";
+import { BackgroundMusic } from "@/lib/audio";
 import { COLOR } from "@/lib/colors";
 import { AmbientOpenScene } from "./scenes/AmbientOpenScene";
 import { CTAScene } from "./scenes/CTAScene";
@@ -74,6 +75,13 @@ export const PromoTeaser: React.FC = () => {
       </Series>
 
       <GrainOverlay opacity={0.03} />
+      <BackgroundMusic
+        src={staticFile("audio/promo-teaser.mp3")}
+        volume={0.5}
+        fadeInFrames={15}
+        fadeOutFrames={45}
+        totalFrames={600}
+      />
     </AbsoluteFill>
   );
 };

apps/videos/src/compositions/x-launch/XLaunchVideo.tsx

@@ -1,6 +1,7 @@
 import type React from "react";
-import { AbsoluteFill, Series } from "remotion";
+import { AbsoluteFill, Series, staticFile } from "remotion";
 import { GrainOverlay } from "@/components/GrainOverlay";
+import { BackgroundMusic } from "@/lib/audio";
 import { COLOR } from "@/lib/colors";
 import { AiShowcaseScene } from "./scenes/AiShowcaseScene";
 import { FeatureBurstScene } from "./scenes/FeatureBurstScene";
@@ -38,6 +39,13 @@ export const XLaunchVideo: React.FC = () => {
       </Series>
 
       <GrainOverlay opacity={0.03} />
+      <BackgroundMusic
+        src={staticFile("audio/x-launch.mp3")}
+        volume={0.4}
+        fadeInFrames={30}
+        fadeOutFrames={60}
+        totalFrames={1050}
+      />
     </AbsoluteFill>
   );
 };